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Solar Water Splitting by Photovoltaic-Electrolysis with a Solar-To-Hydrogen Efficiency Over 30%

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Solar Water Splitting by Photovoltaic-Electrolysis with a Solar-To-Hydrogen Efficiency Over 30% Lawrence Berkeley National Laboratory Recent Work Title Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30% Permalink https://escholarship.org/uc/item/5wr3t7qs Journal Nature Communications, 7 Authors Jia, J Seitz, LC Benck, JD et al. Publication Date 2016-10-31 DOI 10.1038/ncomms13237 Peer reviewed eScholarship.org Powered by the California Digital Library University of California ARTICLE Received 6 May 2016 | Accepted 9 Sep 2016 | Published 31 Oct 2016 DOI: 10.1038/ncomms13237 OPEN Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30% Jieyang Jia1,*, Linsey C. Seitz2,*, Jesse D. Benck2,*, Yijie Huo1, Yusi Chen1, Jia Wei Desmond Ng2,3, Taner Bilir4, James S. Harris1 & Thomas F. Jaramillo2 Hydrogen production via electrochemical water splitting is a promising approach for storing solar energy. For this technology to be economically competitive, it is critical to develop water splitting systems with high solar-to-hydrogen (STH) efficiencies. Here we report a photo- voltaic-electrolysis system with the highest STH efficiency for any water splitting technology to date, to the best of our knowledge. Our system consists of two polymer electrolyte membrane electrolysers in series with one InGaP/GaAs/GaInNAsSb triple-junction solar cell, which produces a large-enough voltage to drive both electrolysers with no additional energy input. The solar concentration is adjusted such that the maximum power point of the photovoltaic is well matched to the operating capacity of the electrolysers to optimize the system efficiency. The system achieves a 48-h average STH efficiency of 30%. These results demonstrate the potential of photovoltaic-electrolysis systems for cost-effective solar energy storage. 1 Department of Electrical Engineering, Stanford University, 350 Serra Mall, Stanford, California 94305, USA. 2 Department of Chemical Engineering, Stanford University, 443 Via Ortega, Shriram Center Room 305, Stanford, California 94305, USA. 3 Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research, Jurong Island 627833, Singapore. 4 Solar Junction, 401 Charcot Avenue, San Jose, California 95131, USA. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to J.S.H. (email: [email protected]) or to T.F.J. (email: [email protected]). NATURE COMMUNICATIONS | 7:13237 | DOI: 10.1038/ncomms13237 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13237 he sustainable nature of solar electricity along with its potential for building extremely high-efficiency solar H2 produc- associated large resource potential and falling costs have tion systems using current state-of-the-art commercially available Tmotivated a rapid increase in the deployment of utility- solar cells and laboratory PEM electrolysers. The device design scale solar electricity generation plants in recent years1. As the presented herein could provide a viable route to implementing installed capacity of photovoltaics (PVs) continues to grow, large-scale solar water splitting installations. cost-effective technologies for solar energy storage will be critical to mitigate the intermittency of the solar resource and to Results maintain stability of the electrical grid2. Hydrogen generation via PV-electrolysis system design. A schematic of the PV-electrolysis solar water splitting represents a promising solution to these system is shown in Fig. 1. The solar cell is a commercially challenges, as H2 can be stored, transported and consumed available triple-junction solar cell manufactured by Solar Junc- without generating harmful byproducts3–8. However, the cost of tion, with an active area of 0.316 cm2. From top to bottom, the H2 produced by electrolysis is still significantly higher than that three subcells of the PV are made of InGaP (Eg ¼ 1.895 eV), GaAs 31 produced by fossil fuels. The Department of Energy has (Eg ¼ 1.414 eV) and GaInNAs(Sb) (Eg ¼ 0.965 eV) respectively . calculated the H2 threshold cost to be $2.00–$4.00 per gallon of The cell was installed on a water-cooled stage, which maintained 9 gasoline equivalent , whereas the most up-to-date reported H2 a cell temperature of B25 °C and was illuminated with white production cost via electrolysis is $3.26–$6.62 per gallon of light from a xenon arc lamp to simulate concentrated AM 1.5D gasoline equivalent10. There are several promising approaches to solar illumination. (See Supplementary Fig. 1 for comparative large-scale solar water splitting, including photochemical, xenon arc lamp and AM 1.5D spectra.) The two PEM electro- photoelectrochemical (PEC) and PV-electrolysis systems8; none lysers consist of Nafion membranes coated with 0.5 mg cm À 2 Pt of these approaches are currently economically viable compared black catalyst at the cathode and 2 mg cm À 2 Ir black catalyst at with today’s technologies3,6,11. the anode. The two electrolysers were connected in series with the To be practical for large-scale deployment, the cost of solar H2 PV cell and a potentiostat, which was used to measure the current generation must be significantly reduced. Previous studies through the circuit. Water was pumped into the anode have predicted that achieving a high solar-to-hydrogen (STH) compartment of the first electrolyser, whereas the cathode of efficiency is a significant driving force for reducing the H2 the first electrolyser had no input flow. The water and O2 effluent generation cost12–14. To date, the highest efficiency demonstrated from the first electrolyser’s anode compartment flowed into the using a PEC water splitting system with at least one anode compartment of the second electrolyser. Likewise, the H2 semiconductor–liquid junction is 12.4% (refs 15,16). Theoretical from the cathode side of the first electrolyser flowed into the studies using a variety of assumptions have predicted that the cathode side of the second electrolyser. The H2 and O2 products maximum attainable efficiency using a tandem PEC water flowing out of the second electrolyser were collected and splitting device is 23–32% (refs 17–20). PV-electrolysis systems quantified, whereas the unreacted water was fed back into a have demonstrated exceptional potential to achieve even higher water reservoir and recycled through the system. The temperature STH efficiencies8,11,21–26. The highest STH efficiency of the electrolysers was held at B80 °C, consistent with standard demonstrated to date, 24.4%, was delivered by a PV-electrolysis operating conditions for industrial water electrolysers21,22. The system using GaInP/GaAs/Ge multi-junction solar cells and system operated continuously for 48 h without interruption. polymer electrolyte electrochemical cells24. For comparison, the Further details of the experimental setup are included in the best multi-junction PV created to date demonstrated a solar-to- Methods section. electricity conversion efficiency of 46.0% under concentrated 27 illumination . In theory, PV-electrolysis systems could poten- System performance. The I–V characteristics of the cell under tially achieve up to 90–95% of the PV efficiency, which could B both 1 sun and concentrated illumination are shown in Fig. 2. allow for PV-electrolysis efficiencies of 57% for a 3J cell and Following a standard method for characterizing concentrated PV B62% for a 4J or 5J cell28. These values indicate that there is significant room for further improvement in the performance of PV-electrolysis system prototypes. Electrolysers H2O The discrepancy between reported STH efficiencies for PV- Unreacted H2O H & O H O recirculated electrolysis devices and stand-alone solar-to-electricity PV 2 2 2 products O2 efficiencies mainly arises from poor matching of the current– H2 – – quantified e e e– voltage (I–V) characteristics of multi-junction PVs with those of 8 water electrolysers . The maximum power-point voltage (VMPP) of a typical commercial triple-junction solar cell is in the range of 2.0–3.5 V under 1–1,000 suns of illumination. However, the h+ h+ thermodynamic minimum voltage required to electrolyse water is only 1.23 V at 300 K (refs 7,29), with practical operating voltages e– in the range of 1.5–1.9 V (refs 7,30). Electrolysing water using a Curent collector Liquid/gas flow Ir blackNafion / Nafion membranePt blackCarbon / Nafion paper H2O Ti mesh + O – voltage in excess of the thermodynamic minimum voltage results 2 e in energy wasted as heat rather than stored in H2 chemical bonds. H Previously, this limitation was overcome by coupling multiple PV 2 and/or electrolyser units in series, to optimize the match between 23,24 InGaP (1.9 eV) the voltage characteristics of these device components , GaAs (1.4 eV) although the efficiencies achieved were still far from optimal. GaInNAs(Sb) (1.0 eV) In this work, we employ a high-efficiency triple-junction solar cell with two series-connected polymer electrolyte membrane e– Photovoltaic (PEM) electrolysers to achieve very high STH efficiency. Our system produces H2 with a 48 h average STH efficiency of 30%, Figure 1 | PV-electrolysis device schematic. The PV-electrolysis system the highest efficiency reported to date for any solar H2 production consists of a triple-junction solar cell and two PEM electrolysers connected system, to the best of our knowledge. This work demonstrates the in series. 2 NATURE COMMUNICATIONS | 7:13237 | DOI: 10.1038/ncomms13237 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13237 ARTICLE a 5.0 b 200 4.5 600 14 180 ) ) –2 4.0 –2 12 160 500 3.5 140 10 3.0 1 Sun 400 42 Suns (mA cm 2 120 2 Area = 0.316 cm y 2.5 8 Area = 0.316 cm V = 2.79 V 100 OC VOC = 3.21 V 300 2.0 J = 13.9 mA cm–2 –2 SC 6 80 JSC = 584 mA cm Current (mA) Current (mA) VMPP = 2.48 V V = 2.91 V 1.5 –2 60 MPP 200 J = 13.4 mA cm –2 MPP 4 JMPP = 566 mA cm 1.0 P = 33.2 mW cm–2 40 –2 Current density (mA cm MPP PMPP = 1,640 mW cm Current densit 2 100 0.5 Fill Factor = 0.858 20 Fill Factor = 0.872 0.0 0 0 0 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Voltage (V) Voltage (V) Figure 2 | PV cell performance.
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